An emerging research area in multi-user MIMO communications is so-called massive MIMO or large-scale MIMO systems. Unlike conventional multi-user MIMO systems that employ less than ten antennas at the base station, a base station in a massive MIMO system is typically equipped with much larger number of antennas, e.g., 64 or more, and serving dozens of users or mobile terminals (e.g., 40) simultaneously.
Compared with conventional MIMO systems, large scale MIMO systems have many benefits. For example, with more antennas employed, each antenna unit can be made smaller and at a lower cost. Also, since a larger number of antenna units can provide more spatial freedom, the base station in any given cell can use the same time and frequency resources to communicate with multiple users simultaneously, which can significantly improve spectral efficiency. The system power efficiency can also be improved because massive antenna units allow for a better spatial orientation between the base station and each user or mobile terminal in the cell for downlink and/or uplink transmissions, which can significantly reduce the transmission power from both the base station and mobile terminal sides. In addition, when there exists a sufficient number of base station antennas, the random channels between each user and the base station can be nearly orthogonal to each other, which can help eliminate inter-cell and inter-user interferences and noises. For the above reasons, it is expected that large scale MIMO systems will be widely used.
Despite the above-mentioned technological advantages, however, current applications of massive MIMO may be constrained by existing transmission schemes. For example, in a cellular system, a public channel is often used to carry important signals from the base station to users, such as synchronization signals, reference signals within the cell, control signals, multimedia broadcast multicast service (MBMS) signals, and the like. Generally speaking, the transmission scheme for the public channel requires an omni-directional and reliable transmission to ensure cell-wide coverage. However, most existing transmission schemes, such as single antenna transmission, cyclic delay diversity (CDD) or space-time block codes (STBC), may be incompatible with a massive MIMO system. For example, under the single-antenna transmission scheme, a single antenna is chosen from the transmitting antennas in the base station for broadcasting signals. In that case, the chosen antenna is equipped with a much more powerful and expensive power amplifier to ensure the same area of coverage as all antennas are used for broadcasting signals. Nonetheless, this scheme would not work in a massive MIMO system because, by using a large number of antennas, each antenna is made small with a much less powerful amplifier for power reduction reasons, and no antenna would be sufficiently powerful to serve as the single antenna mentioned above. Similarly, many other transmission schemes, including the space-time block coding (STBC) and cyclic delay diversity (CDD) schemes widely used in the LTE systems, may not be applicable in massive MIMO systems. The increase in the number of antennas in a massive MIMO system can cause design challenges such as significantly increased pilot overhead.
Therefore, a need exists for a transmission scheme that can provide omni-directional coverage in massive MIMO systems without causing additional pilot overhead or compromising system efficiency.